The recombinant cell and the flooded cell are two different types of commercially available lead acid battery designs. Both types include adjacent positive and negative electrodes that are separated from each other by a porous battery separator. The porous separator prevents the adjacent electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow between adjacent positive and negative plates.
One type of recombinant battery, a VRLA battery, typically includes an absorptive glass mat (AGM) separator composed of microglass fibers. While AGM separators provide high porosity and uniform electrolyte distribution, they offer little control over the oxygen transport rate or the recombination process. Furthermore, AGM separators exhibit low puncture resistance, which is detrimental to the operation of the VRLA battery in a high vibration environment, such as within an automobile. Low puncture resistance is problematic for two reasons: (1) the incidence of short circuits increases and (2) manufacturing costs are increased because of the fragility of the AGM sheets. One attempt to produce a VRLA battery having improved separator puncture resistance and oxygen recombination entailed the use of a polyethylene separator having a gelled electrolyte design. The battery included a sulfuric acid electrolyte and cross-linked silica particles that formed a three-dimensional gel.
In the second type of lead acid battery, the flooded cell battery, only a small portion of the electrolyte is absorbed into the separator. Flooded cell battery separators typically include porous derivatives of cellulose, polyvinyl chloride, organic rubber, and polyolefins. More specifically, microporous polyethylene separators are commonly used because of their ultrafine pore size, which inhibits dendritic growth while providing low electrical resistance, good oxidation resistance, and excellent flexibility.
Thus most flooded lead acid batteries include polyethylene separators. The term “polyethylene separator” is something of a misnomer because these microporous separators require large amounts of precipitated silica to be sufficiently acid wettable. The volume fraction of precipitated silica and its distribution in the separator generally control its electrical properties, while the volume fraction and orientation of polyethylene in the separator generally control its mechanical properties.
Most types of commercially available precipitated silica are available as powders with the as-received individual particles having diameters in a range of approximately 5–50 micrometers. As shown in FIG. 1, a silica particle 10 is comprised of multiple interconnected silica aggregates 20, each of which has a diameter of about 0.1 to about 0.2 micrometer. Each individual silica aggregate 20 is comprised of multiple covalently bonded primary particles 30, each of which has a diameter of about 20 nanometers.
Silica particles 10 derive their porosity from the interstices between and within silica aggregates 20. The degree of hydrogen and/or covalent bonding between silica aggregates 20 determines the friability of the commercially available precipitated silica. The amount of hydrogen and/or covalent bonding between silica aggregates 20 can be influenced by the precipitation and drying processes used to manufacture the commercially available precipitated silica.
Commercially available precipitated silica is typically combined with a polyolefin, a process oil, and various minor ingredients to form a separator mixture that is extruded at an elevated temperature through a slot die to form an oil-filled sheet. The oil-filled sheet is calendered to its desired thickness and profile, and the majority of the process oil is extracted. The sheet is dried to form a microporous polyolefin separator and is slit into an appropriate width for a specific battery design.
During battery manufacture, the separator is fed to a machine that forms “envelopes” by cutting the separator material and sealing its edges such that an electrode can be inserted to form an electrode package. The electrode packages are stacked such that the separator acts as a physical spacer and an electronic insulator between positive and negative electrodes. An electrolyte is then introduced into the assembled battery to facilitate ionic conduction within the battery.
The primary purposes of the polyolefin contained in the separator are to (1) provide mechanical integrity to the polymer matrix so that the separator can be enveloped at high speeds and (2) to prevent grid wire puncture during battery assembly or operation. Thus, the hydrophobic polyolefin preferably has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high puncture resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator web, thereby lowering the electrical resistivity of the separator. In the absence of silica, the sulfuric acid would not wet the hydrophobic web and ion transport would not occur, resulting in an inoperative battery.
Consequently, the silica component of the separator typically accounts for between about 60% and about 80% by weight of the separator, i.e., the separator has a silica-to-polyethylene weight ratio of between about 2.7:1 and about 3.5:1. One reason a large amount of silica is required is that the silica particles are not completely broken down into their individual aggregates during the extrusion process, thereby providing insufficient silica dispersion throughout the separator web. Increasing the amount of silica in the web relative to the amount of mechanical integrity-stabilizing polyolefin results in low electrical resistance at the expense of puncture resistance.
In response to the commercial demand for increased puncture resistance, some separator manufacturers have attempted to decrease the concentration of silica in the polyethylene separator. However, this undesirably increases the electrical resistance. Further, these separators displayed inadequate acid wettability. One attempt to increase the acid wettability of the low concentration silica separators involved applying a combination hydrophobic/hydrophilic surfactant coating to the web. While the surfactant successfully increased the acid wettability and decreased the electrical resistance of the separator, it complexed with the soluble lead oxides of the battery and degraded to form a black scum that interfered with battery operation. Moreover, the use of a surfactant introduced a significant additional processing expense.
It is therefore desirable to cost-effectively produce a microporous polyethylene separator having a material composition that provides increased puncture resistance and high oxidation resistance while maintaining a low electrical resistance.
A related, but separate, concern involves the addition of an antioxidant to the separator. Antioxidants are added to the polyethylene separator to protect its mechanical integrity by preventing polyethylene degradation during processing and use. These antioxidants are typically added to the separator formulation before extrusion to reduce oxidation and molecular weight reduction of the polymer matrix during the extrusion process. It has been discovered in many cases that most of the added antioxidant material is not present in the separator web following solvent extraction. In fact, only about 20% to about 30% of the initial antioxidant concentration is present in the final separator. Much of this loss occurs during solvent extraction of the process oil from the separator web. In addition, some of the antioxidant is thermally degraded during the extrusion and extraction processes.
It is therefore further desirable to produce a microporous polyethylene battery separator that more efficiently maintains and distributes the antioxidant throughout the web.